1. Field of the Invention
This invention relates to water-soluble copolymers. More particularly, it relates to water-soluble copolymers useful for enhanced oil recovery.
2. Description of the Prior Art
Heretofore, there have been known, as viscosity improvers used for enhanced oil recovery, water-soluble polymers, such as copolymers of acrylamide, acrylic acid or salt thereof, and N-sulfoethyl- or sulfopropyl-acrylamide or -methacrylamide. Certain of such polymers, however, have tendency of being degraded under mechanical shearing or resulting in poor viscosity-increasing property when contacted with water containing traces of impurities.